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carbon steel manufactured by CQ Steel. The galvanized, tinned, and plastic composite steel plates made of the original plates are used extensively for decorative panels, general dust pipes, drums, instrument cases, switchboxes, and train cars, among other uses. The target control composition is shown in Table I. The S and P contents are usually fixed during production below 0.020 and 0.025 wt%, respectively, with the aim of reducing longitudinal cracks on the slab surface. This type of carbon steel (Q235B) is manufactured by numerous steel companies worldwide owing to the high market demand and simple production process. The main production issues are related to the control of the castability of the molten steel while reducing costs. The production route for Q235B followed by CQ Steel is currently as follows: converter smelting composition adjustment by sealed argon bubbling (CAS) continuous casting. Although this route ensures smooth production, ladle furnace (LF) and Ca treatments are usually employed to solve occasional issues. In order to ensure a high sulphur capacity, the refining slag compositions should have a CaO/SiO2 ratio of 3–4, a SiO2 content fixed at 17 wt%, and an Al2O3 content of approximately 20 wt% (the area delimited by circles in Figure 1). This results in a slag melting temperature of approximately 1700oC. In this case, the following issues appear: (i) the content of the high-melting-point Al2O3 inclusions is higher and the castability of the molten steel is unstable; (ii) the clogging ratio of the nozzle is close to 20% in the all production heats, thereby resulting in the molten steel not satisfying the casting process; and (iii) the presence of longitudinal cracks on the slab surface, typically caused by the above reasons, leading to slab offline inspection and cleaning and the product not meeting the cast-rolling compact layout and hot charging requirements. The LF refining process was employed in other companies in the past (Peng et al., 2007; Xu et al., 2009; Zhi et al., 2004; Li, He, and Zhang, 2010). However, the CQ Steel plant used CAS processing, and more stringent requirements at the operating level have been proposed. With this background, this investigation was aimed at achieving high efficiency and low-cost production by adjusting the refining slagging process to ensure cleanliness of the steel.

Effect of different refining slag systems on the cleanliness of molten steel for carbon structural steel